This invention relates generally to rotating electrical machinery and more particularly to a leading power factor induction motor drive system.
Induction and synchronous motors are essentially constant speed electrical machines. Many industrial, military and traction applications, however, require electric drives with wide speed variations. The requirement of having a wide operating speed range prevented both synchronous and induction alternating current motors from being practical for wide speed range variable speed drives until the recent advent of high-power, solid-state electronic components which have been used to construct line and forced-commutated inverters for variable-voltage, variable-frequency power supplies for AC motor drives. Yet today, even the most successful AC motor drives cannot compete, in terms of initial cost, with traditional DC motor drives used in most variable speed motor applications. The reason for the high cost of AC motor drive systems is that the least expensive inverter, the line-commutated type, and the least expensive motor, the squirrel-cage induction type, are fundamentally incompatible in terms of their reactive power requirements.
A characteristic of AC circuits is that the corresponding voltage and current waveforms are not always in phase. The product of the out-of-phase voltage and current components is generally referred to as reactive power or reactive volt-ampere. When the phase of the current lags or leads the voltage, the corresponding power factor is referred to as a lagging power factor or a leading power factor, respectively.
AC induction motors typically operate at a lagging power factor, while line-commutated inverters can function only if the load they supply has a leading power factor. A static inverter, which is capable of supplying lagging power factor loads, must have auxiliary means to force the commutation of the solid-state switching devices which comprise the inverter; hence the name forced-commutated inverter also referred to as self-commutated inverter. The requirement for separate commutation circuitry makes the forced-commutated inverter significantly more expensive than a line-commutated inverter having a comparable power rating.
Synchronous motors are capable of operating at a leading power factor and, therefore, are compatible with line-commutated inverters. Below a minimum speed, however, synchronous machines fail to generate sufficient voltage to line-commutate the thyristors in the inverter, which necessitates a special start mode for such machines. In the past, two methods have been used to start synchronous motors operated in conjunction with line-commutated inverters. One method is to provide motor starting by means of a forced-commutated inverter that operates only at low speeds when the motor is being started and is disconnected once the speed is sufficiently high to assure line-commutation. A second method is to selectively pulse current into sets of the synchronous machine winding through the inverter, which causes the machine to accelerate until reaching a speed where the generated voltage is sufficient forline-commutation. In the second starting mode, inverter commutation is achieved by quenching the current in the DC link. The shortcomings of the two foregoing methods of starting a synchronous motor are that the first method requires a separate forced-commutated inverter and that the second method produces a relatively low starting torque. The low starting torque results from limitations on the amount of pulse-power that can be transferred through the DC link. In some applications, such as traction drives, the maximum torque demand occurs at starting, which severely limits the utility of starting a synchronous machine by selectively pulsing the machine windings through the inverter. A further problem associated with synchronous motors is the requirement for rotor excitation, which necessitates either slip-rings or a complicated brushless system.
The aforementioned difficulties with synchronous motors illustrate the need in the art for the development of a novel AC motor drive system which provides the rugged construction of squirrel-cage motors and the simplicity of line-commutated inverters.